1. Field of the Invention
The invention relates to a method for machining a wall of a cylinder of an internal combustion engine, in particular the invention relates to a method for coating the cylinder wall in which this is roughened beforehand in order to improve the adhesion of the coating.
2. Description of Related Art
For reasons of weight, internal combustion engines in motor vehicles are regularly designed with an engine block made of light metal, in particular aluminum. However, it can prove problematic here that, due to the tribological properties of the light metal, in particular the comparatively poor resistance to wear, the inner walls of the cylinders formed by the engine block are only inadequately suitable as running surfaces for the pistons.
In order to avoid these problems, in many internal combustion engines with an engine block made of light metal cylinder liners made of, for example, grey cast iron are used, as a result of which the advantages of the light metal (low specific weight) can be combined with those of the grey cast iron (good tribological properties).
Alternatively, it is known for the cylinder walls of the light metal engine blocks to be provided with a coating in order to realize the desired tribological properties for the running surfaces. Such a coating is regularly realized by melting the coating material and spraying it onto the cylinder walls.
It is of particular importance in the coating of cylinder walls to achieve a good adhesion of the coating layer to the base material. For this purpose it is known for the cylinder walls of the engine block to be roughened prior to coating in order to increase the contact surface area between the layers. In many cases an attempt is also made to form undercuts in the roughened cylinder walls into which the melted coating material can penetrate in order to achieve a form-locking and force-locking connection with the cylinder walls following hardening.
Various different possible ways of roughening the cylinder walls are known.
These include, on the one hand, the methods known as “high pressure blasting”, in which the cylinder walls are blasted with solid particles or a liquid under high pressure. As a result, particles of the wall material are broken out and the wall material is plastically deformed at individual points. In particular, blasting with corundum, which is delivered by means of a gas jet, as well as blasting with water or an emulsion are widely used. The gas jet or the liquid are thereby delivered from the nozzles which are used under a very high pressure of, regularly, 1500 bar to 3500 bar. One disadvantage of high pressure blasting is the rapid wear on the nozzles due to the high pressure, possibly in combination with the discharge of an abrasive material. In addition, adherence to tolerances of form and position is difficult and therefore associated with considerable expense and effort. And finally, this involves a high consumption of the medium used, which in some cases can also be associated with considerable effort expended on its preparation for recycling.
It is also known for the cylinder walls to be roughened mechanically. This can for example be achieved through rough turning, in which grooves or corrugations are formed in the cylinder walls in a cutting or non-cutting process using one or more tools.
It is for example known from DE 10 2009 027 200 B3, for the purpose of roughening a cylinder wall, in a first method step to introduce a helical groove with, in particular, rectangular or trapezoidal cross section into the cylinder wall. This can be achieved using both cutting and non-cutting processes. In a second step, the rib running helically between the grooves is then, in sections, plastically deformed in a radial direction in order to form undercuts. In order to introduce the groove a tool is used which has a single blade, the tool being both driven so as to rotate as well as being moved axially along the longitudinal axis of the cylinder.
It can prove disadvantageous in the known mechanical methods that the introduced profile can display a comparatively pronounced notch effect which occurs in particular with a cutting method of machining due to the resulting sharp-edged groove profile. Also, the tools are often unable to machine the ends of the cylinders completely. And finally, with the mechanical methods it is generally necessary subsequently to clean the machined cylinder walls in order to eliminate machining residues and to obtain oil- and dirt-free surfaces, which are necessary for a high-quality coating.
Alternative methods for preparing cylinder walls for a subsequent coating include chemical and electrical roughening, brushing and spraying with adhesive primer.